Florida
Karst.
Florida is built on limestone that water slowly dissolves. Beneath the sand and clay, voids open and grow for years before anything shows at the surface. This is the ground that makes Florida sinkhole country, and the reason a real foundation repair here has to reach rock.
Water that eats stone.
Rain picks up carbon dioxide from the air and more as it soaks through soil, turning into a weak acid. Limestone dissolves in it.
Under most of Florida lies thick limestone, and inside that limestone moves the Floridan aquifer, one of the most productive groundwater systems in the country. As slightly acidic water works through fractures and weak layers in the rock, it widens them into channels and cavities. Geologists call this kind of dissolved, cavity-riddled landscape karst.
On top of the rock sits a cover of sand and clay that can be a few feet thick or more than a hundred. That cover is what decides how the ground fails. Thick sand trickles down into voids a little at a time. Clay can bridge a void and hold, sometimes for years, until the bridge gives way all at once.
The Southwest Florida Water Management District notes that where limestone lies more than about 100 feet down under thick clay and sand, sinkholes are rare. Where the rock is shallower, and especially where people have built on top of it, sinkholes are far more common. That is why the west central counties of Hillsborough, Pasco and Hernando are often called Sinkhole Alley.
For a building owner, the practical point is simple. The problem is rarely the soil the slab sits on. It is what is happening in the rock thirty, sixty or a hundred feet below it, and in the soil slowly raveling down into that rock.
Three kinds of sinkhole.
The thickness and makeup of the cover over the limestone decide which kind forms, how fast, and how much warning anyone gets.
Solution
Where limestone is at or just under the surface, water dissolves it from the top down. These form slowly and continuously, usually as bowls that hold water.
Cover-subsidence
Under thick sand with little clay, grains filter down into cavities as they form. The surface settles gradually, and the sinkholes usually stay small, a few feet across and deep.
Cover-collapse
Where clay in the cover arches over a cavity, the void can keep growing unseen. When the clay bridge fails the collapse is abrupt, and its size is set by the cavity below.
What sets it off.
Dissolution takes centuries. The collapse usually comes after something changes the load on the cover or the water holding it up.
Heavy rain
Saturated soil adds weight to the roof over a cavity.
Drought
Falling groundwater removes the buoyant support under that roof. Drought followed by heavy rain is the worst combination.
Pumping
Large withdrawals drop aquifer pressure locally and quickly.
Construction
New loads, retention ponds, changed drainage, heavy traffic and drilling vibration all play a part.
Eleven nights of freeze.
During a record cold snap in January 2010, farms in eastern Hillsborough County pumped groundwater onto their crops to protect them from frost, night after night. The Southwest Florida Water Management District reported that the aquifer dropped about 60 feet. Within days the District received almost 140 sinkhole complaints and more than 750 complaints of dry wells.
The District later declared the Dover and Plant City Water Use Caution Area to manage frost and freeze pumping in the region.
Warning signs.
None of these prove a sinkhole on their own. Plenty of them come from ordinary settlement or drainage. Together, or appearing quickly, they are a reason to investigate.
- 01New cracks in walls, floors, block or pavement, or cracks opening in the ground
- 02Doors and windows that suddenly stick or no longer close
- 03Fence posts, trees or structures beginning to lean or sag
- 04Freshly exposed roots or soil at the base of posts, trees or the foundation
- 05Rainwater ponding where it never collected before
- 06Small circular patches of wilting or dying vegetation
- 07Cloudy water in a nearby well
- 08A shallow depression forming in the yard
You cannot see it from the yard.
Karst is diagnosed below ground. A proper sinkhole investigation combines direct borings with geophysics that maps between them.
SPT borings
A split spoon is driven by a 140 lb hammer and the blows are counted, per ASTM D1586. Very low counts, weight of hammer readings and lost drilling fluid point to raveled soil and voids. See the boring log on the FRN homepage.
Ground penetrating radar
Radar pulses map disturbances in the cover soils across the whole site, showing where to drill and how far an anomaly extends.
Electrical resistivity
Resistivity profiles image deeper, showing where clay is thin, where the rock surface dips and where water-filled or soil-filled voids may sit.
Two very different coverages.
Florida Statute 627.706 separates a catastrophic collapse from ordinary sinkhole damage, and the difference decides whether many homeowners are covered at all.
Catastrophic ground cover collapse
Every insurer writing property insurance in Florida must cover it. The statute requires geological activity that results in all four of these:
- The abrupt collapse of the ground cover
- A depression in the ground cover clearly visible to the naked eye
- Structural damage to the covered building, including the foundation
- The structure being condemned and ordered vacated by the governmental agency authorized to do so
Sinkhole loss
Insurers must make sinkhole loss coverage available, but the owner has to buy it for an additional premium, and the insurer may inspect the property first.
Most sinkhole damage never meets all four collapse conditions. Cracked walls and a settling slab over a raveling zone are real structural problems, but without sinkhole loss coverage they are often not insured. Check your policy before you need it.
General information, not legal advice. Read the current statute and talk to your agent or attorney about your own policy.
Florida karst events.
Three Florida events that show how karst fails, from a city block to a single bedroom.
In May a sinkhole roughly an acre across opened in Winter Park, taking a house, a swimming pool and part of a car dealership. It filled with water and is now Lake Rose.
Cover-collapse at city scale. The void had been growing long before anyone saw a crack.
Freeze protection pumping dropped the aquifer about 60 feet. Almost 140 sinkhole complaints and more than 750 dry well complaints followed.
People can trigger in days what geology prepared over thousands of years.
A sinkhole opened beneath a bedroom at night and a man was lost. The house was demolished and the hole filled.
Cover-collapse gives little warning. Investigation, not appearance, separates a sound lot from a dangerous one.
Reach the rock, fill the void.
A karst repair has two jobs: carry the building past the weak and raveled zone, and stabilize the soil that is moving into the rock. Many repairs do both.
Foundation Masters
Steel push piers and driven pipe piles installed under the footing to bearing below the active zone, with civil forensic engineering behind the design. Florida CA# 30969.
foundationmasters.com →Foundation Waterproofing 101
Water drives most karst triggers at the house scale. Collection lines, sump systems and drainage that keep concentrated water off the cover soils. CGC 1539326.
foundationwaterproofing101.com →Seeing the signs?
Tell us what is happening at the property and whether you already have an inspection, boring log or proposal. We reply by email.
Send a question →- Southwest Florida Water Management District, Sinkholes brochure: formation, sinkhole types, triggers and warning signs
- Southwest Florida Water Management District, District Responds to the Freeze of 2010: Dover and Plant City figures
- Southwest Florida Water Management District, Dover/Plant City Water Use Caution Area
- Florida Statutes 627.706: catastrophic ground cover collapse and sinkhole loss coverage
- U.S. Geological Survey, What is a sinkhole?
The Foundation Repair Network publishes general educational information about geology, soils and foundation performance. It is not an engineering evaluation of any specific property or legal advice. Conditions vary site by site; consult a qualified engineer before making repair decisions.